The constantly increasing demand for high data rates in wireless communication networks continues to put pressure on service operators to evolve their existing wireless networks. Several approaches have been implemented to provide higher data rates. One approach is to increase the density of macro base stations in the network and to increase the cooperation between macro base stations. While increasing macro base station density and cooperation provides increased data rates, the added cost and delay associated with adding macro base stations often makes this option undesirable.
Another approach involves deploying smaller base stations within an already existing macro layer grid, i.e., building a heterogeneous network. Smaller base stations are more cost-efficient than macro base stations and can typically be deployed in a short time span. The macro layer is able to service users moving at high speed, as well as service wider areas where the demand for high data rates is less, while the smaller base stations are able to provide increased functionality. For example, the use of smaller base stations in the macro layer grid allows service operators to customize deployment of smaller base stations to service areas having higher density of users that require higher data rates, i.e., hotspots. However, the dense deployment of macro base stations and smaller base stations causes the problem of increased signaling complexity due to frequent handovers of users moving at high speeds.
One instantiation of a heterogeneous network that decreases signaling complexity is a shared cell heterogeneous deployment. In a shared cell, the macro base station and smaller base stations within macro coverage area share the same cellular identification (ID) such that, from a mobile user equipment (UE) perspective, the macro and smaller base stations appear as one cell. In particular, with respect to long term evolution (LTE) networks, a central evolved node B (“CeNodeB”) is connected to a main, i.e., high power, radio that defines the macrocell. The CeNodeB is also connected to lower power radio units (“RUs”) or reception/transmission (“R/T”) point. Each R/T point will typically have at least two antennas such as a main antenna and a diversity antenna and also shares the same cell ID used by the CeNodeB. The shared cell approach avoids the proliferation of cell IDs and does not require multiple carriers.
However, in a shared cell, the radio propagation conditions are such that in many instances a UE will not be able to hear the downlink signal from one or more R/T points. While this situation presents the potential for either spectral reuse within the shared cell, or power saving and interference reduction within the shared cell, the CeNodeB is required to determine which UE-R/T point pairs have a viable communication channel, i.e., are hearable in both uplink and downlink directions. In a shared cell heterogeneous network, the CeNodeB can distinguish the R/T point using uplink signals from each R/T point on the common public radio interface (“CPRI”) line on which the UE uplink signals appear, as discussed above. This allows uplink hearability for any UE-R/T point combination to be determined by assessing the uplink signal specific to the pair in question. In time division duplex (“TDD”) systems, due to the uplink and downlink reciprocity, the uplink hearability assessment also indicates downlink hearability. However, for frequency division duplex (“FDD”) systems, the uplink hearability does not indicate downlink hearability status.